Supermassive black holes are found at the centers of galaxies. Although they are prevalent in the present-day universe, it is unclear when they first formed, and how many of them exist now. Distant SMBHs are identified as quasars, which shine as gas accretes onto them, but previous studies have been sensitive only to the very rare most luminous quasars, and thus the most massive black holes. The new discoveries probe the population of SMBH with masses characteristic of the most common black holes seen in the present-day universe, and thus shed light on their origin.
To choose the candidate quasars for the study, the research team led by Yoshiki Matsuoka (Ehime University) used data taken with a cutting-edge instrument, “Hyper Suprime-Cam” (HSC). Mounted on the Subaru Telescope of the National Astronomical Observatory of Japan, on the summit of Maunakea in Hawaii, the HSC is particularly powerful due its gigantic field-of-view of 1.77 deg2 (seven times the area of the Full Moon). The HSC team is carrying a survey of the sky using 300 nights of telescope time, spread over five years. With these data the researchers selected the quasars the analyses of which resulted in the findings of supermassive black holes.
Moreover, they carried out an intensive observational campaign to obtain spectra of those candidates, using the Subaru Telescope, the Gran Telescopio Canarias, and the Gemini telescope. Researcher Kazushi Iwasawa from the Institute of Cosmos Sciences of the UB, has been the principal researcher of the observations conducted with the GTC in this second phase, in which they discovered about a third of new quasars.
The survey has revealed 83 previously unknown very distant quasars; together with the 17 quasars already known in the survey region, Matsuoka and collaborators found that there is roughly one supermassive black hole in each cube a billion light years on a side.
Furthermore, the astronomers discovered quasars are about 13 billion light-years away from the Earth; in other words, we are seeing them as they existed before. The time elapsed since the Big Bang to that cosmic era is only 5 % of the present cosmic age (13.8 billion years), and it is remarkable that such massive dense objects were able to form so soon after the Big Bang. The most distant quasar discovered by the team is 13.05 billion light-years away, a similar distance to the second most distant supermassive black hole ever discovered.
On the other hand, results of the research imply a reconsideration of the hypothesis on re-ionization of hydrogen in cosmos. It is widely accepted that the hydrogen in the universe was once neutral, but was “re-ionized” (i.e., split into its protons and electrons) around the epoch when the first generation of stars, galaxies, and SMBHs were born, in the first few hundred million years after the Big Bang. This is a milestone of cosmic history, but it is still not clear what provided the incredible amount of energy required to cause the re-ionization. A compelling hypothesis suggests that there were many more quasars in the early universe than detected previously, and it is their integrated radiation that re-ionized the universe. However, the number density measured by the HSC team clearly indicates that this is not the case; the number of seen quasars is significantly less than needed to explain the re-ionization. Re-ionization was therefore caused by another energy source, most likely numerous galaxies that started to form in the young universe.
With the obtained results so far, the team aims to find more distant supermassive black holes and reveal the period in which the first one appeared in the universe.
The research team, led by Yoshiki Matsuoka, is built up by 48 international astronomers. Researchers whose relevant role in the individual phases of the project are noteworthy are Nobunari Kashikawa (The University of Tokyo), Michael Strauss (Princeton University), Masafusa Onoue (Max Plank Institute for Astronomy), Kazushi Iwasawa (ICCUB) and Tomotsugu Goto (National Tsing Hua University). Results of this research study have been published in the journals Astrophysical Journal Letters, The Astrophysical Journal Supplement Series, Publications of the Astronomical Society of Japan and The Astrophysical Journal.
 “Discovery of the First Low-luminosity Quasar at z > 7”, Matsuoka et al., The Astrophysical Journal Letters, 872 (2019), 2
 “Subaru High-z Exploration of Low-luminosity Quasars (SHELLQs). V. Quasar Luminosity Function and Contribution to Cosmic Reionization at z = 6”, Matsuoka et al. 2018, The Astrophysical Journal, 869 (2018), 150
 “Subaru High-z Exploration of Low-luminosity Quasars (SHELLQs). IV. Discovery of 41 Quasars and Luminous Galaxies at 5.7 ≤ z ≤ 6.9”, Matsuoka et al., The Astrophysical Journal Supplement Series, 237 (2018), 5
 “Subaru High-z Exploration of Low-Luminosity Quasars (SHELLQs). II. Discovery of 32 quasars and luminous galaxies at 5.7 < z ≤ 6.8”, Matsuoka et al., Publications of the Astronomical Society of Japan, 70 (2018), S35
 “Subaru High-z Exploration of Low-luminosity Quasars (SHELLQs). I. Discovery of 15 Quasars and Bright Galaxies at 5.7 < z < 6.9”, Matsuoka et al., The Astrophysical Journal, 828 (2016), 2